Glass-ceramic articles are produced through the controlled crystallization of glass articles. In general, a glassforming batch, to which a nucleating agent is commonly added, is melted and this melt cooled to a uniform glass body.
The glass body is then subjected to a heat treatment schedule which usually comprises two phases. First, the glass is heated somewhat above the transformation range of the glass to cause the development of nuclei therein. Thereafter, the nucleated glass is heated to a higher temperature, normally above the softening point of the glass, to cause the growth of crystals on the nuclei. Since the crystallization occurs on the innumerable nuclei developed during the nucleation step, the crystals are uniformly fine-grained and homogeneously dispersed in a glassy matrix.
The crystals generally comprise the preponderance of the article and, therefore, endow the article with properties more similar to those of the crystal phase than those of the original glass. However, since the body is crystallized in situ from a glass, it is free of voids and non-porous.
Silicon semiconductors are more efficient in display or photovoltaic devices if the grains are large. Silicon melts at 1417° C., and it can be subsequently crystallized to the largest grains when held just below its melting point. This is because nucleation is inefficient at such high temperatures but grain growth is maximized. Since the growth of these crystallized films initially requires application of a film of molten silicon to a substrate surface, it is important that the substrate material itself be inert, able to withstand high temperature, and have a coefficient of expansion (CTE) that approximately matches that of silicon. If silicon is melted on an inert substrate and subsequently recrystallized near its melting point, it will remain as a coherent large-grained film upon cooling to room temperature providing the substrate approximately matches the silicon in CTE. The glass-ceramics of this invention can be tailored to match or be very close to the CTE of silicon. Moreover, certain suboxide semiconductors that also display photovoltaic behavior, e.g. reduced titanates and certain tantalates, require high temperatures (above 1300° C.) to be effectively sintered. Therefore, films of these materials require a substrate which can withstand sintering temperatures above 1400° C. Further, there is also a need for ceramic materials which can replace superalloy metals in certain engine components such as those used in aircraft engines and in diesel turbochargers. Silicon nitride is such a candidate, but it is difficult to fabricate and inherently expensive. Accordingly, there is a need for a high temperature glass ceramic material that can be utilized as an inert substrate for thin film crystal silicon production, can be tailored to match the CTE of silicon, can withstand temperatures of as high as 1450° C., is resistant to oxidation, and is capable of being precision formed in the fluid glassy state by pressing techniques or by simple casting into graphite or other molds.